This invention pertains to jet-operated pumps for removing or diluting dross from the surface of a bath of molten metal, and more particularly a pump in which a gas is introduced into the pump conduit removing or diluting the dross, in the same direction (a "jet") as that of the moving metal/dross.
This invention further pertains to a pump in which the gas is introduced into the pump conduit in the form of a gas jet through a convergent/divergent nozzle. This invention also pertains to a composite material jet-operated pump for removing or diluting dross from the surface of a bath of molten metal. Steel utilized in the automotive, construction and appliance industries and the like is formed in very thin strips (0.015 to 0.060 inches thick), which are then passed through a molten bath of either aluminum (aluminizing), zinc (galvanizing) or aluminum/zinc (galvalume). The strip width usually ranges from 30 to 70 inches. To avoid the formation of oxides on the strip'surface that are detrimental to the coating quality, the strip is delivered to the molten bath from a nitrogen/hydrogen filled furnace through a tubular housing (snout), also filled with the same gas.
Because of the extremely large dimensions of the equipment, and in spite of efforts to prevent air leaks into the furnace, small air leaks occur, generating ferrous oxides (Fe.sub.2 O.sub.3, FeO, etc.).
When the steel strip enters the bath, a chemical process occurs in which the aluminum or zinc in the bath reacts with the iron oxides to form dross, aluminum oxides (Al.sub.2 O.sub.3) and/or zinc oxides (ZnO). The released free iron settles to the bottom of the molten metal pot. Because of their slightly lower density in the molten metal, alumina (Al.sub.2 O.sub.3) and zinc oxide (ZnO), remain in suspension or float to the surface. This dross accumulates in the area enclosed by the snout. Since the dross is generally a very hard ceramic and usually contains large particulates that adhere to the steel strip being processed, a defective coating, having a poor appearance and high rejection rates may result.
In U.S. Pat. No. 5,683,650, a bubble-operated pump is disclosed in which an inert gas is introduced into a conduit having its inlet end disposed beneath the surface of a bath of molten metal. The conduit is generally U-shaped with a pair of upright legs. One leg has an inlet opening in the dross layer. The other leg has an outlet opening disposed outside of the snout housing which contains the dross. In a dross-diluting version, the positioning of the inlet and outlet is reversed. A gas is introduced in the outlet leg in a direction at right angles to the motion of the dross/metal. Upon being introduced into the conduit, the gas creates spaced apart bubbles which induce the flow of metal/dross toward the outlet opening. This pump is a reliable and inexpensive mechanism having no moving parts for removing dross from the surface of a molten metal bath enclosed in a delivery snout.
Moreover, the pump comprises a one-piece tubular conduit having an inlet side for receiving molten metal and an outlet side for discharging the metal. A source of an inert gas such as nitrogen (or argon) is connected in the outlet side of the conduit to produce the stream of rising bubbles. As the nitrogen bubbles upwards toward the surface, it creates a suction effect in the inlet side of the conduit generating a flow of metal in the same direction. However, in certain instances, the gas creates a fluidic blockage, retarding the flow.